The present invention relates generally to compacting polymer powder and more particularly to compacting a semicrystalline polymer-metal powder blend to a form a preform having high strength and low electrical resistivity.
Many avionic components are currently made of aluminum, but interest in reducing weight has promoted the development of lighter-weight thermoplastic polymers to replace the aluminum. The parts for avionic components must, however, meet stringent electromagnetic interference (EMI) shielding requirements. Consequently, the thermoplastic polymers for these EMI shielding applications must be made to be electrically conductive. Metal filler is often added to the polymers for this purpose. In addition to being lighter in weight than aluminum, the resulting polymer-metal composites exhibit improved corrosion resistance and are cheaper to manufacture.
Many of the polymer composites currently used are formed by melt processing methods such as extrusion, and injection and compression molding. Aside from being costly, these methods of manufacture are undesirable because they result in composites with reduced mechanical properties and electrical conductivity, and typically require high metal filler loadings. These composites also experience high shear rates during these processes which causes undesirable metal particle segregation, particularly at the composite surfaces and along flow lines.
Certain types of metal-filled thermoplastic polymer composites are currently formed by the method of compaction. The thermplastics used are categorized as either amorphous or semicrystalline. Amorphous thermoplastics are usually compacted at a temperature near the glass-transition temperature. The semicrystalline thermoplastics compacted to-date, such as high density polyethylene, have been compacted at a temperature greater than their glass-transition temperatures. These particular semicrystalline thermoplastic polymers have such low glass-transition temperatures, that compaction above those temperatures is not uneconomical.
Polyether-etherketone (hereinafter referred to as PEEK) is a desirable candidate for polymer composite materials because of its excellent mechanical properties, and its good thermal stability and solvent resistance. However, due to its high melting point, high melt viscosity, and low crystallation rate, conventional melt processing methods are not economically attractive. Previously, PEEK has not been considered a candidate for compaction because of its high glass-transition temperature and the belief that successful compaction must take place above that temperature.